On the Challenge of Convection Permitting Precipitation Simulations: Results from Idealised Experiments

Limited-area convection-permitting climate models (CPMs) with horizontal grid-spacing less than 4km and not relying on deep convection parameterisations (CPs) are being used more and more frequently. CPMs represent small-scale features such as deep convection more realistically than coarser regional climate models (RCMs) with deep CPs. Because of computational costs CPMs tend to use smaller horizontal domains than RCMs. As all limited-area models (LAMs), CPMs suffer issues with lateral boundary conditions (LBCs) and nesting. We investigated these issues using idealised Big-Brother (BB) experiments with the LAM COSMO-CLM. Grid-spacing of the reference BB simulation was 2.4 km. Deep convection was triggered by idealised hills with driving data from simulations with different spatial resolutions, with/without deep CP, and with different nesting frequencies and LBC formulations. All our nested idealised 2.4-km Little-Brother (LB) experiments performed worse than a coarser CPM simulation (4.9km) which used a four times larger computational domain and yet spent only half the computational cost. A boundary zone of >100 grid-points of the LBs could not be interpreted meteorologically because of spin-up of convection and boundary inconsistencies. Hosts with grid-spacing in the so-called grey zone of convection (ca. 4 - 20km) were not advantageous to the LB performance. The LB's performance was insensitive to the applied LBC formulation and updating (if smaller or equal 3-hourly). Therefore, our idealised experiments suggested to opt for a larger domain instead of a higher resolution even if coarser than usual (~5km) as a compromise between the harmful boundary problems, computational cost and improved representation of processes by CPMs.